This invention relates to a method for the production of an investment shell mold to be used in producing by grain-oriented casting a Ni-base super alloy suitable for fabrication of a precision cast article.
Heretofore, investment shell molds for casting an alloy containing only a small amount of active elements such as Al or Ti have been produced by repeatedly applying on a mold-producing pattern alternate layers of a slurry having zircon or fused silica flour blended in a silicate binder and layers of grains of zircon, fused silica, molochite, or mullite and subsequently firing the applied layers of the aforementioned slurry and grains at 800.degree. to 1,000.degree. C. The mold produced by this method, however, has free silica on the cavity surface thereof. When this mold is used in producing a grain-oriented super alloy having a high Al or Ti content, since the molten alloy and the mold are held in contact with each other for a long time during the grain-oriented solidification at elevated temperatures, the free silica on the cavity surface of the mold is reduced to Si by Al or Ti and passes into the molten alloy, with the result that the silicon degrades the mechanical properties of the cast article at elevated temperatures. The mold itself possesses a poor high-temperature strength of not more than 20 kgf/cm.sup.2 at 1,400.degree. C. and tends to deform and, therefore, cannot be effectively used in such a casting as a grain-oriented casting which keeps the mold in contact with the molten alloy of highly elevated temperature for a long time.
Formerly, a group of inventors including the present inventor perfected a method for the production of an investment shell mold free from the drawback described above and, therefore, suitable for the fabrication of a grain-orientedly solidified super alloy. This invention was filed under U.S. patent application Ser. No. 625,895.
This method is characterized by the steps of preparing a slurry by dispersing powdered oxide of at least one element selected from the group consisting of magnesium, aluminum, zirconium, hafnium, yttrium, calcium, lanthanum, cesium, barium, and silicon in a solution of an organic soluble cellulose derivative in an organic solvent, applying the slurry on the surface of a mold-producing pattern, and then applying thereon a slurry having a powdered refractory substance mixed with a high-temperature binder, thereby forming a refractory slurry layer.
The mold produced by this method amply endures the molten metal pressure of an alloy being cast therein. It has .alpha.-Al.sub.2 O.sub.3 forming the entire cavity surface thereof. The SiO.sub.2 which is produced from the high-temperature binder is not allowed to come into contact with the molten alloy.
This method nevertheless suffers from the following disadvantage.
In forming a face coat of the mold inactive to the melt of super alloy, this method uses an organic cellulose as a binding material. During the removal of the used pattern, which is generally effected by a treatment in an autoclave, there is the possibility that the face coat alone will be peeled off the mold.
The present invention has been produced for the purpose of overcoming this drawback.